1. Field of the Invention
This invention relates to non-destructive evaluation of solid objects and more particularly, to ultrasonic wave inspection systems for the detection of cracks in solid objects.
1. Description of the Prior Art
Reactor coolant pumps are an integral part of the reactor cooling system. These pumps are responsible for pumping coolant into the core during the operation of a nuclear power plant. The failure of a reactor coolant pump would result in a decreased amount of coolant getting into the core and could cause exceedingly high temperatures to occur in the core. If more than one failed (which is highly improbable) or if the pump impeller were to break the integrity of the coolant system, then a major loss of coolant could occur. This could potentially lead to core overheating. Therefore, the failure of a reactor coolant pump is a major safety concern.
The reactor coolant pump is a high horsepower unit. The pump shaft, which is usually fabricated from heavy stainless steel, must transform the torque of the motor into pumping power and it is therefore subjected to very high stresses. Therefore, pump shafts are susceptible to fatigue cracking due to these large cyclic stresses. Since the shaft has so many diameter changes and usually has a threaded region, any defects in these regions can act as stress risers and ultimately to crack initiation sites.
In the history of these coolant pumps, no failures have been reported until 1985. At that time, the failure of a reactor coolant pump shaft at a nuclear power plant was reported. As a result of this failure, several utilities which had similar pumps were required to perform inspections on their pumps and provide data on the integrity of their pumps to the Nuclear Regulatory Commission. The results of these inspections showed several shafts with indications of cracking. However, after these pumps were disassembled and the pump shafts inspected with liquid penetrant, the cracks were not confirmed with the liquid penetrant. This led to the need for improving the reliability of ultrasonic inspection for the detection of cracks in the pump shaft.
A reactor coolant pump shaft is a long, large diameter, cylindrical body, which has many variations in diameter along its length. The pump shaft is directly mated to the pump impeller and sustains a great amount of stress from converting the torque of the high horse power motor into pumping force.
Pump shafts can be inspected in several ways which require various stages of pump disassembly. One way is to disassemble the pump so that the pump shaft is totally accessible. In this mode, liquid penetrant techniques can be used to detect cracking. However, disassembly of the pump is costly and very time consuming. Therefore, it is desirable to utilize non-destructive evaluation (NDE) techniques that can be used with the pump in situ and require minimal pump disassembly.
Ultrasonics is the most often used NDE method for inspecting pump shafts in situ. Two ultrasonic techniques have been used; namely, 0-degree longitudinal wave and the high angle longitudinal wave. Each technique requires different stages of pump disassembly.
When the 0-degree longitudinal wave technique is used, the cover of the pump must be removed which exposes the flat end of the pump shaft. To utilize the high angle longitudinal wave technique, further disassembly of the pump is required to remove seals so that almost the entire length of the pump shaft is exposed. To date, use of both of these techniques have lead to some inconclusive inspection results. In utilizing the conventional 0-degree longitudinal wave technique, a transducer with a frequency in the range of 1-2.25 MHz with a diameter of approximately 2.54 cm (1 inch) to 3.81 cm (1.5 inches) is used to scan over the surface of the exposed end of the pump shaft. The pump shaft may have a center hole which is not scanned over. There are two problems associated with this inspection. First, the small diameter transducer does not provide sufficient energy into the pump shaft. The energy input is related to the size of the transducer. Secondly, the small transducer produces mode-converted signals which can be difficult to interpret, especially since the transducer beam is striking the pump shaft wall asymmetrically.
When the high angle longitudinal wave technique is used, more pump disassembly is required so that the transducer beam can intersect the regions of interest. This technique has been susceptible to giving indications due to metallurgical variations in the pump shaft which are not detrimental to the pump shaft condition. Therefore, the high angle longitudinal wave technique requires too much pump disassembly and has been observed to give false indications due to metallurgical variations in the pump shaft.